1. Field of Invention
The present invention relates to a control device, and more particularly to a motor control device.
2. Related Art
The control technology for motors has been well developed. The methods of controlling a motor using a circuit to change the rotation rate of the motor or to control the rotating direction of the motor have universally existed in various fields of motor control.
As shown in FIG. 1, a conventional motor control device 1 for controlling such as a fan motor 14 includes a pulse width modulation (PWM) generating circuit 11, a DC voltage generating circuit 12 and a drive chip 13. The PWM generating circuit 11 generates a pulse width modulation signal PWM1 and inputs the pulse width modulation signal PWM1 to the DC voltage generating circuit 12. The DC voltage generating circuit 12 converts the pulse width modulation signal PWM1 into a DC voltage signal Vdc1. The DC voltage signal Vdc1 is compared with a high-frequency triangular wave signal Stri built in the drive chip 13, and a drive signal Dri1 is then generated to drive the fan motor 14 to rotate. Further referring to FIG. 2, when the duty cycle of the pulse width modulation signal PWM1 varies, the converted DC voltage signal Vdc1 varies. Different drive signals for controlling the rotation rate of the fan motor 14 are modulated after the DC voltage signal Vdc1 is compared with the high-frequency triangular wave signal Stri of the drive chip 13.
As shown in FIG. 2, the duty cycle of the drive signal Dri1 is constant (does not vary with time). A current waveform I1 detected from the power terminal of the motor control device 1 is shown in FIG. 3 when the fan motor 14 is controlled under a low rotation rate. A current waveform I2 detected from the power terminal of the motor control device 1 is shown in FIG. 4 when the fan motor 14 is controlled under a high rotation rate.
As shown in FIG. 3, the current waveform I1 generates an inrush current during the phase of the rotation-rate waveform FG outputted from the fan motor 14 is changed. The inrush current can enable the fan motor 14 to change the polarity and rotate. However, a larger rotating torque is generated corresponding to the inrush current, and the noises occur during the polarity of the fan motor 14 is changed under a low rotation rate.
As shown in FIG. 4, the duty cycle of the current waveform I2 does not vary during the phase of the rotation-rate waveform FG outputted from the fan motor 14 is changed. The current increases as the rotation rate increases, and the noises occur during the polarity of the fan motor 14 is changed under a high rotation rate.
The above-mentioned drawbacks about the noises will not only restrict the application of the motor but weaken the purchase desire. Therefore, it is imperative to provide a motor control device to reduce the noise caused by the rotation of the motor.